As is known, steering systems for vehicles generally comprise a steering wheel able to activate in rotation a steering column, which, in turn, following rotation imposed by the steering wheel, imparts steering on the vehicle wheels.
Generally, in particular in heavy vehicles, such as agricultural tractors, a hydrostatic steering unit is used, which facilitates the vehicle steering and makes the steering maneuver easier.
In practice, the hydraulic power steering system, of known type, also known as Orbitrol, generally comprises a gerotor which is connected to the base of the steering column activated by the steering wheel and is such as to send oil into one or more hydraulic pistons connected to the hubs of the wheels and thus command steering thereof as a function of the rotation imposed on the steering wheel.
In heavy vehicles, the cubic capacity of the hydraulic piston which acts on the hubs of the wheels is necessarily very high, for example around 1000 cc.; therefore the cubic capacity of the hydraulic power steering system that has to actuate the hydraulic piston must also exhibit a high cubic capacity so as to enable displacement of a sufficient quantity of fluid (for filling the chambers of the hydraulic piston) with a relatively modest number of revolutions of the steering wheel.
In cases of faults or malfunctioning, the manual rotation of the steering wheel is the only force responsible for activating the hydraulic power steering which transforms into a pump. The activating of hydraulic power steering systems of large cubic capacity is very hard for the operator and the steering is effectively inhibited.
In the attempt to obviate these drawbacks, when a large cubic capacity of the hydraulic power steering is required, a hydraulic system with two gerotors is used, in which during the normal advancement step the gerotors are substantially connected and guarantee the ideal cubic capacity, while in a case of malfunctioning or faults they are disconnected and the operator, by rotating the steering wheel, commands one alone which thus enables steering the vehicle.
A further drawback which is encountered in steering systems with hydrostatic steering units is that the position of the spokes of the steering wheel is never defined and fixed with respect to the reference position which they should have according to their design; in practice the position of the steering wheel is never fixed with respect to the position of the wheels being steered.
In practice, it has been found that in a first step, with the vehicle wheels in a straight direction, the spokes of the steering wheel are in a first reference position, while following use of the steering system when the wheels are returned to the straight position, the spokes of the steering wheel are orientated differently with respect to the first reference position, by a quantity which is not predictable and variable in the presence fluid-dynamic leakage of the hydraulic power steering system.
In this situation, therefore, it has been found that the spokes of the steering wheel, due to the uncertainty and indeterminateness of the position thereof, do not enable easy viewing of the control panel, for example provided with monitoring instruments of the state of functioning of the vehicle and/or of an instrument fascia of the commands when positioned behind the steering wheel; further, for this reason, contrarily to what occurs in motor cars, there are no logos or writings identifying the vehicle manufacturer on the steering wheel, or any other writing, as in use they would frequently be upside-down, even with the wheels in a straight configuration.
Other types of steering systems, known as “steer by wire”, include mechanically disconnecting the steering column from the steering lever mechanisms of the wheels and delegating the translation of the rotations imposed on the steering wheel to an electrical actuator which, commanded by an electronic control unit attached to an angular position sensor placed on the steering column, activates the wheel-steering lever mechanisms according to a predetermined and settable transmission ratio which can vary also according to some functioning parameters of the vehicle, such as for example the advancing velocity thereof, the advancement direction or other presettable parameters.
To manage a fault or malfunctioning of the electrical part, to continue functioning correctly in an emergency, these steer by wire mechanisms require appropriate backups both for the electrical actuators and for the supply batteries, with a consequent increase in system costs.
An aim of the present invention is to obviate the above-mentioned drawbacks of the prior art, with a solution that is simple, rational and relatively inexpensive and which preserves the advantages of the steering system with a hydraulically-assisted steering system, among which the manageability of the vehicle even in a case of faults or malfunctioning of the electronic command board, and at the same time can exhibit all the advantages of the “steer by wire” systems, among others a correct positioning of the steering wheel into the zero position and a settable manoeuvring comfort of the steering wheel, for example variable according to the functioning parameters of the vehicle.
These aims are attained by the characteristics of the invention as reported in the independent claim. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.